Sectional density - help me understand

[Goosey]

I continually see comments on this forum about sectional density being some kind of a predictor of terminal bullet performance. Such as a 140g 6.5 bullet providing better penetration than a 140g .277 bullet, and therefore being superior on big game. As the SD is no more than weight divided by the square of the diameter, I can grasp how a non-deforming solid of minimally higher SD would out-penetrate a bullet with a lesser SD...in theory. But since the vast majority of hunting appropriate bullets expand in a manner much more determined by what they hit than what they looked like getting there, how can small differences in SD even be quantified in terms of actual penetration? Seems to me the unpredictable differences in expanded diameter when contacting hide/meat/bone, especially when squared, instantly obviate any theoretical advantage the higher SD bullet might have had. Sure, I can see how a doubling of SD would predict better penetration, all other things being equal, but a few ticks three places to the right of the decimal point? Has anyone demonstrated - in a repeatable manner - that slight increases in SD, with equal expanding bullets, equate to more penetration IN something? At a given velocity and bullet construction, I'd guess bullet weight means more to penetration than SD.

You can't look at sectional density and say purely from this that one specific expanding bullet will penetrate more than another. Even in the same product line (eg Nosler Ballistic Tip, Sierra Gameking, etc) bullets of different weight or caliber can have big differences in their construction. If everything is equal then higher SD equalling more penetration is true in theory, but everything being equal is rare.

People also forget that SD changes as soon as your bullet hits something. A 180 gr .308" bullet has twice the SD of a Brenneke slug. But as soon as that bullet expands and sheds weight, that SD could be reduced to one-sixth of the original value. How does it compare to the Brenneke slug then?

One advantage SD gives every time is in helping out with the ballistic coefficient, a good one which consists of two things working together: the SD and the shape.

 

[someguy2800]

Sectional density is only a meaningful number when comparing two bullets of the same construction and similar weight. I have never been that concerned about sectional density because it does not tell me what I need to know and it does not scale very well. For example a 100 grain .243", and 350 grain .458" bullet have almost the same sectional density. If sectional density was really as good of a predictor of penetration as people say it is then those two would have similar penetration at the same velocities, but obviously that is false. I am more concerned about bullet weight and bullet construction than I am with sectional density.

 

[JumboJVT]

As some may have surmised, I do understand how little SD matters until the difference become orders of magnitude. Its the repeated contention that small variations do matter that I was intending to impugn. Most seem to agree. Yet others seem to have missed the "small differences in SD" and "a few ticks three places to the right of the decimal point" references in the OP.

 

[Hog huntin Harry]

A 150 grain .284 PSP has a S.D. of 0.266
A 150 grain .308 PSP has a S.D. of 0.226

If both bullets are of the same construction and impact velocity, the .284 bullet will penetrate deeper. The .308 bullet won't penetrate as far as the .284 bullet, but it's greater expanded frontal area will produce a larger wound channel. So it's a difference between hitting more of the game animal or doing more damage to what you do hit.

 

[d2wing]

The only time it has mattered to me is comparing bullets of similar construction in the same caliber. For instance comparing a 124 gr 7.62 bullet from a 7.62x39 to a 170 gr 30-30 bullet traveling at close to the same speed. Both are 30 caliber but the 30-30 will have more energy and likely greater penetration on similar targets. Again assuming both are soft point expanding bullets. Perhaps more true if both were FMJ or solids.

 

[John#2]

Assuming identical BHNs, bullet construction, and normal velocities, if a 44 magnum weighing 220 grains impacts an animal like an elk, it will not penetrate as deeply as a 220 grain 30 caliber bullet going the same velocity on impact >>> because that 30 caliber bullet has significantly greater sectional density.

Assuming both rifles weigh the same, congratulations. You just bought yourself more penetration with the 30 caliber without biting off more recoil.

This is exactly why the 7mm and 30 calibers have always been so popular with hunters. They fall in that sweet spot among velocity, total bullet mass, recoil, and penetration stemming from adequate to excellent sectional density.

Side note: high sectional density roughly correlates to better ballistic coefficients. Compare in general terms the trajectory of .429 to .308 bullets leaving the muzzle at the same velocity.

 

[Varminterror]

Assuming identical BHNs, bullet construction, and normal velocities, if a 44 magnum weighing 220 grains impacts an animal like an elk, it will not penetrate as deeply as a 220 grain 30 caliber bullet going the same velocity on impact >>> because that 30 caliber bullet has significantly greater sectional density.

Assuming both rifles weigh the same, congratulations. You just bought yourself more penetration with the 30 caliber without biting off more recoil.

This is exactly why the 7mm and 30 calibers have always been so popular with hunters. They fall in that sweet spot among velocity, total bullet mass, recoil, and penetration stemming from adequate to excellent sectional density.

This is a big miss in my book.

When we talk about popular 7mm’s and 30 cals, specifically a 30 cal as you’ve referenced which could handle a 220grn bullet, you’re talking about a lot more recoil in the 30 caliber rifle. Most guys aren’t going to run a 220 in as small of case as a 30-06, but it can be done. You’re talking about 55-60grn of powder pushing the bullet up around 2400-2450fps, opposed to a 44mag pushing a 220 class bullet to around 1850-1900fps over 22-25grn of powder. That’s over 60% greater recoil momentum for the 30-06 than the 44mag.

Hunting with a 44mag buys cheap ammo and reduced recoil, at the cost of diminished potential range. But there’s no magic to it.

 

[John#2]

“Assuming . . . going the same velocity on impact“

Of course, greater velocities result in greater recoil for the same bullet mass. But that was outside my gedankenexperiment.

I stipulated same velocities, which in the world of cast lead bullets is entirely possible, if not probable.

In any case, a thought experiment showing underlying physics principles doesn’t have to have a real life analogue to make its point.